Nitrogen oxide decomposing element and nitrogen oxide decomposing apparatus including the same

ABSTRACT

A nitrogen oxide decomposing element and a nitrogen oxide decomposing apparatus can perform a treatment at a relatively low temperature without using a material, which is suspected to have influence on the environment and human body, as an oxidant or a catalyst. There is proposed a nitrogen oxide decomposing element  1  including a conductive solid electrolyte film  2  for selectively allowing a hydrogen ion to pass through, a first electrode layer  3  made of an electronic conductivity base material and a catalyst for accelerating anodic oxidation, a second electrode layer  4  made of an electronic conductivity basematerialandacatalystforacceleratingcathodicreduction, and a platinum group catalyst  6  supported by a porous metal oxide  5  disposed to be adjacent to the second electrode layer  4.  A low-power consumption nitrogen oxide decomposing apparatus which can efficiently use electric energy is obtained by locating a nitrogen oxide sensor  14  in the vicinity of the platinum group catalyst  6  supported by the metal oxide  5,  and controlling the magnitude of a current flowing between the first and the second electrode layers  3  and  4  and the energization time by a power source/control device  15  in accordance with the concentration of nitrogen oxide detected by the nitrogen oxide sensor  14.

TECHNICAL FIELD

The present invention relates to a nitrogen oxide decomposing elementfor decomposing and removing nitrogen oxide and a nitrogen oxidedecomposing apparatus including the same.

BACKGROUND ART

A nitrogen oxide purging system for decomposing and removing nitrogenoxide has various uses, such as an exhaust gas treatment of anautomobile, a distributed cogeneration system, and air cleaning of anenclosed space such as a long tunnel or a factory, and increased demandis expected into the future. As a conventional technique to removenitrogen oxide, for example, there is JP-A-2002-153755 as patentdocument 1. This patent document 1 discloses a method of purgingnitrogen oxide NOx by using ammonia as a reductant. There is disclosedthat in this method, an ammonia molecule (NH₃molecule) donates anelectron through an adjacent noble metal to nitrogen monoxide (NO)adsorbed on the noble metal, and as a result, the nitrogen monoxide (NO)becomes apt to dissociate into a (N atoms) become a nitrogen molecule(N₂ molecule). At this time, the temperature of a flowing gas is 400° C.

As another prior art to remove nitrogen oxide, there is JP-A-11-342313as patent document 2. The patent document 2 discloses a method oftreating a harmful substance-containing gas and an apparatus therefor,in which ozone as a highly safe oxidizer is used to be capable oftreating a gas containing harmful substances such as various organiccontaminants, malodorous components, and bacteria. The patent document 2proposes that after ozone is added to and mixed with the gas containingthe harmful substances, the gas is passed through an adsorbent layerfilled with high silica adsorbent for adsorbing the ozone and foradsorbing the harmful substances, so that the harmful substances in thegas are made harmless by the action of the ozone.

However, the method proposed in patent document 1 has problems that itis not desirable to use ammonia in consideration for the environment andhuman body, and further, it is necessary to perform the treatment at ahigh temperature of 400° C., it is difficult to perform the treatmentadapted to variations in the concentration of nitrogen oxide, and thetreatment of unreacted ammonia is required. Also in the method of patentdocument 2, in the case where the adsorbed nitrogen oxide is desorbed toreuse the adsorbent, it is necessary to perform the treatment at a hightemperature of 300 to 400° C.

The invention has been made to solve the foregoing problems, and a firstobject thereof is to propose a nitrogen oxide decomposing elementcapable of performing a treatment at a relatively low temperaturewithout using a material, which is suspected to have influence on theenvironment and human body, as an oxidant or a catalyst.

A second object thereof is to propose a nitrogen oxide decomposingapparatus using the above nitrogen oxide decomposing element.

DISCLOSURE OF THE INVENTION

A nitrogen oxide decomposing element of the invention includes aconductive solid electrolyte film for selectively allowing a hydrogenion to pass through, a first electrode made of an electronicconductivity base material disposed on a part of a surface of theconductive solid electrolyte film and a catalyst for accelerating anodicoxidation, a second electrode made of an electronic conductivity basematerial disposed on the other part of the surface of the conductivesolid electrolyte film and a catalyst for accelerating cathodicreduction, and a platinum group catalyst supported by a porous metaloxide disposed to be adjacent to the second electrode.

According to the nitrogen oxide decomposing element of the invention,the nitrogen oxide decomposing element is obtained in which a treatmentcan be performed at a relatively low temperature (60° C. to 80° C.)without using a material, which is suspected to have influence on theenvironment and human body, as an oxidant or a catalyst.

Besides, a nitrogen oxide decomposing apparatus of the inventionincludes the nitrogen oxide decomposing element, a frame holding this, agas supply ports for supplying an anode gas and a cathode gas into thisframe, a gas exhaust port for exhausting the gases in the frame tooutside, and a power source for applying a DC voltage between the firstand the second electrodes.

According to the nitrogen oxide decomposing apparatus of the invention,the nitrogen oxide decomposing apparatus is obtained in which atreatment can be performed at a relatively low temperature (60° C. to80° C.) without using a material, which is suspected to have influenceon the environment and human body, as an oxidant or a catalyst.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a nitrogen oxide decomposingapparatus of embodiment 1 of the invention.

FIG. 2 is a view showing a current effect of the nitrogen oxidedecomposing apparatus of embodiment 1 of the invention with respect tothe removal of nitrogen monoxide at 60° C.

FIG. 3 is a view showing a current effect of the nitrogen oxidedecomposing apparatus of embodiment 1 of the invention with respect tothe removal of nitrogen monoxide at 70° C.

FIG. 4 is a view showing a current effect of the nitrogen oxidedecomposing apparatus of embodiment 1 of the invention with respect tothe removal of nitrogen monoxide at 80° C.

FIG. 5 is aview showing the current effects of the nitrogen oxidedecomposing apparatus of embodiment 1 of the invention with respect tothe removal of nitrogen oxide at 60° C., 70° C. and 80° C.

FIG. 6 is a schematic view showing a nitrogen oxide decomposingapparatus of embodiment 3 of the invention.

FIG. 7 is a view showing the operation and effect of the nitrogen oxidedecomposing apparatus of embodiment 3 of the invention.

FIG. 8 is a schematic view showing a nitrogen oxide decomposingapparatus of embodiment 4 of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the invention will be described withreference to the drawings.

EMBODIMENT 1

FIG. 1 is a schematic view showing a nitrogen oxide decomposing elementof embodiment 1 of the invention and a structure of a nitrogen oxidedecomposing apparatus.

The nitrogen oxide decomposing apparatus of embodiment 1 includes anitrogen oxide decomposing element 1 and a frame 7 for housing it. Thenitrogen oxide decomposing element 1 includes a conductive solidelectrolyte film 2 for selectively allowing a hydrogen ion to passthrough, a first electrode layer 3 made of an electronic conductivitybase material disposed to be in contact with a part of a surface of thesolid electrolyte film 2 and a catalyst (hereinafter referred to as ananodic catalyst) for accelerating anodic oxidation, a second electrodelayer 4 made of an electronic conductivity base material disposed to bein contact with the other part of the surface of the solid electrolytefilm 2 and a catalyst (hereinafter referred to as a cathodic catalyst)for accelerating cathodic oxidation, and a platinum group catalyst 6supported by a porous metal oxide 5 disposed to be adjacent to thesecond electrode layer 4.

The solid electrolyte film 2 has plane surfaces 2 a and 2 b opposite toeach other, and the first electrode layer 3 and the second electrodelayer 4 are respectively formed to be in close contact with the planesurfaces 2 a and 2 b.

In embodiment 1, as the solid electrolyte film 2, Nafion 117 (tradename) made by Dupont Company is used. As each of the first electrodelayer 3 and the second electrode layer 4, a feeding body is used inwhich a porous platinum thin film layer and a porous titanium arelaminated by plating, and a titanium surface of the porous titanium isfurther plated with platinum, and the platinum plating of the surfacehas functions of the anodic catalyst and the cathodic catalyst.

In embodiment 1, although platinum is used as both the anodic catalystand the cathodic catalyst, it is not necessary that these are the same,and as the anodic catalyst, in addition to platinum, it is possible touse iridium, iridium oxide or the like. In the case where the firstelectrode layer 3 and the second electrode layer 4 respectively do notinclude the anodic catalyst and the cathodic catalyst, unless a veryhigh voltage is applied to the nitrogen oxide decomposing element 1, thereactions of anodic oxidation and cathodic reduction are not quicklypromoted, and therefore, these catalysts are indispensable.

Further, the porous metal oxide 5 having the function of occluding andconcentrating nitrogen oxide is disposed to be in contact with thesecond electrode layer 4, and platinum as the platinum group catalyst 6is supported by the metal oxide 5 at a weight ratio of 1 wt %. That is,the metal molecules of the platinum group catalyst 6 are contained inpores of the porous metal oxide 5. As a method of causing the porousmetal oxide 5 to support the platinum group catalyst 6, a porous body isimmersed in a solution containing a platinum component, for example,achloroplatinic acid (H₂PtCl₆) solution, and is heated up to about 400°C., so that platinum is supported by the porous body. In embodiment 1,hydrogen-terminated zeolite is used as the porous metal oxide 5. Thezeolite includes aluminum oxide and silicon oxide as components, and isdenoted by using a general formula W_(m)Z_(n)O_(2n).sH₂O. Where, in thisgeneral formula, W denotes one of sodium (Na), calcium (Ca), potassium(K), barium (Ba) and strontium (Sr), Z denotes silicon (Si)+aluminum(Al), the ratio of silicon (Si) to aluminum (Al) is larger than 1(Si/Al>1), and s is not constant.

The metal oxide 5 used in embodiment 1 is an acidic oxide or anamphoteric oxide. The acidic oxide reacts with a base (alkali) to formsalt, and a high oxidation number oxide of transition metal, forexample, titanium oxide (TiO₂) belongs to this. On the other hand, theamphoteric oxide is one oxide which indicates an acidity to a base andindicate a basicity to an acid, and for example, aluminum oxide (Al₂O₃)belongs to this.

The nitrogen oxide decomposing element 1 constructed of the solidelectrolyte film 2, the first electrode layer 3, the second electrodelayer 4, and the platinum group catalyst 6 supported by the metal oxide5 is held by the frame 7. The frame 7 is constructed of a circular framebody 7 a, an upper cover body 7 b airtightly joined to its upper endface through an O-ring 8 a, and a lower cover body 7 c airtightly joinedto its lower end face through an O-ring 8 b. The space in the frame 7 isdivided into one at the side of the first electrode 3 and one at theside of the second electrode 4 by the solid electrolyte film 2, and anupper treatment chamber 7 d and a lower treatment chamber 7 e areformed. An anode gas supply port 9 and an anode gas exhaust port 10 areattached to the upper cover body 7 b, and a mixed gas of water vapor(H₂O) and nitrogen (N₂) is supplied into the upper treatment chamber 7 dthrough the anode gas supply port 9. The nitrogen (N₂) may be replacedby air. A cathode gas supply port 11 and a cathode gas exhaust port 12are attached to the lower cover body 7 c, and a mixed gas of nitrogenmonoxide (NO) as nitrogen oxide, oxygen (O₂) and helium (He) is suppliedinto the lower treatment chamber 7 e through the cathode gas supportport 11.

Reactions at the respective places of the nitrogen oxide decomposingelement 1 in embodiment 1 are as follows.

Reaction (anodic reaction) on the first electrode layerH₂O→2H⁺+½O₂+2e⁻  (chemical formula 1)Reaction (cathodic reaction) on the second electrode layer2H⁺+½O₂+2e⁻→H₂O  (chemical formula 2)2H⁺+2e⁻→H₂  (chemical formula 3)2H⁺+2NO+2e⁻→N₂+H₂O+½O₂  (chemical formula 4)2H⁺+2e⁻+½NO+¼O₂→¼N₂+H₂O  (chemical formula 5)2H⁺+2e⁻+⅔NO+⅓O₂→⅓N₂O+H₂O  (chemical formula 6)N₂O+2H⁺+2e⁻→N₂+H₂O  (chemical formula 7)Reaction on the platinum group catalyst (cathode side)H₂+2NO→N₂+H₂O+½O₂  (chemical formula 8)2H₂+NO+½O₂→2/1N₂+2H₂O  (chemical formula 9)3/2H₂+NO+½O₂→½N₂O+3/2H₂O  (chemical formula 10)N₂O+H₂→N₂+H₂O  (chemical formula 11)

The function of the nitrogen oxide decomposing element 1 of embodiment 1will be described with reference to the chemical formula 1 to thechemical formula 11. A nitrogen gas containing water vapor is suppliedthrough the anode gas supply port 9 into the upper treatment chamber 7 dat the side of the first electrode layer 3, and is made to come incontact with the first electrode layer 3. At the same time, a gascontaining nitrogen oxide is supplied through the cathode supply port 11into the lower processing chamber 7 e at the side of the secondelectrode layer 4, and is made to come in contact with the secondelectrode layer 4. In this state, the energization of a DC power source13 is started, and when a DC voltage is applied between the first andthe second electrode layers 3 and 4, the first electrode layer 3 becomesan anode, the second electrode layer 4 becomes a cathode, and the anodicoxidation and cathodic reduction occur by the actions of the catalystsprovided in the respective layers.

First, in the first electrode layer 3 as the anode, as indicated by theformula (chemical formula 1), the water molecule (H₂O) is electrolyzedand the hydrogen ion (H⁺) is produced. The hydrogen ion is moved to theside of the second electrode 4 through the solid electrolyte film 2,reacts with the nitrogen oxide (NO) on the second electrode layer 4, andas indicated by (chemical formula 4) and (chemical formula 5), thenitrogen oxide is electrochemically reduced and decomposed into thenitrogen molecule (N₂) and water molecule. Besides, as indicated by theformula (chemical formula 6), in the case where nitrous oxide (N₂O) isproduced, it is further reduced and decomposed as indicated by theformula (chemical formula 7), and is decomposed into the nitrogenmolecule and water molecule. On the platinum group catalyst 6 at thecathode side, the hydrogen molecule (chemical formula 3) produced on thesecond electrode 4 and the nitrogen oxide react with each other by theaction of the platinum group catalyst 6, and the nitrogen oxide ischemically reduced and decomposed as indicated by the formulas (chemicalformula 8) to (chemical formula 11).

As stated above, in the nitrogen oxide decomposing element 1 ofembodiment 1, the nitrogen oxide is decomposed by two kinds ofreactions, that is, the chemical reductive reaction (formulas (chemicalformula 8) to (chemical formula 11)) in which only molecules areinvolved, and the electrochemical reductive reaction (formulas (chemicalformula 1) to (chemical formula 7)) in which ions are involved. There isa case where these two kinds of reactions occur at the same time, andthere is a case where only one of them occurs.

FIGS. 2, 3 and 4 show current effects with respect to the removal ofnitrogen oxide in the case where in the nitrogen oxide decomposingapparatus constructed as described above, the area of each of thereaction surfaces, that is, the first and the second electrode layers 3and 4 is 0.8 cm², nitrogen monoxide, as the nitrogen oxide, having aconcentration of 1000 ppm and contained in helium (He) gas is suppliedat a flow rate 10 ml/min, and a DC constant current is supplied betweenthe first and the second electrode layers 3 and 4. Here,the nitrogenoxide (NOx) is the sum of NO and NO₂. In FIGS. 2, 3 and 4, the ratios ofNOx decomposed at 60° C., 70° C. and 802 C. are obtained by analyzinggases exhausted from the cathode gas exhaust port 12 by an analyzer.

That is, they are obtained by (ratio % of decomposed NOx)={(NOxconcentration (0.1%) at the supply port)−(NOx concentration % at theexhaust port)}÷(NOx concentration (0.1%) at the supply port)×100. Here,the NOx concentration is the sum of the concentrations of NO and NO₂.

In FIG. 5, the vertical axis indicates the nitrogen monoxideconcentration at the exhaust port and the horizontal axis indicates thecurrent, and FIGS. 2, 3 and 4 are shown as a graph. A curved line Aindicates the case of 60° C. of FIG. 2, a curved line B indicates thecase of 70° C. of FIG. 3, and a curved line C indicates the case of 80°C. of FIG. 4. As shown in FIG. 5, at any temperatures, as the currentdensity increases, the decomposition removal effect of the nitrogenoxide increases, and in the range of the current density of 15 mA/cm² orhigher, about 80% of the nitrogen oxide was removed. Since the nitrogenoxide removal effect at 80° C., which is highest among these, isinferior to the results at 60° C. and 70° C., it is indicated that evenin the case where the temperature is raised up to 80° C. or higher, ahigher effect is not obtained. That is, the nitrogen oxide decomposingapparatus in this embodiment can sufficiently exhibit the effect at arelatively low temperature of 60° C. to 80° C.

As stated above, according to embodiment 1, there is proposed thenitrogen oxide decomposing element 1 including the first electrode layer3 and the second electrode layer 4 respectively disposed on the opposedplane surfaces 2 a and 2 b of the surface of the solid electrolyte film2, and the platinum group catalyst 6 supported by the porous metal oxide5 disposed to be adjacent to the second electrode layer 4. This nitrogenoxide decomposing element 1 is held by the frame 7, the gas containingwater vapor as the anode gas is supplied through the anode gas supplyport 9 into the frame 7 and the gas containing nitrogen oxide as thecathode gas is supplied through the cathode gas supply port 11, and theDC voltage is applied between the first and the second electrode layers3 and 4 by the DC power source 13. As a result, the nitrogen oxidedecomposing apparatus is obtained which can perform the treatment at arelatively low temperature (60° C. to 80° C.) without using a material,which is suspected to have influence on the environment or human body,as the oxidant or the catalyst.

In embodiment 1, although the example has been described in which thehydrogen-terminated zeolite is used as the porous metal oxide 5 forsupporting the platinum group catalyst 6, the same effect is obtainedeven in the case where a metal oxide containing at least one componentof titaniumdioxide, zirconium dioxide, aluminum oxide, silicon oxide,magnesium oxide and tin oxide is used. Besides, in embodiment 1,although the example has been described in which platinum is used as theplatinum group catalyst 6 supported by the metal oxide 5, the sameeffect is also obtained by iridium or palladium.

EMBODIMENT 2

A nitrogen oxide decomposing element of embodiment 2 (not shown) is suchthat a mixed layer including an electronic conductivity base material, asolid electrolyte film, a platinum group catalyst and a cathodiccatalyst is disposed between the solid electrolyte film 2 of thenitrogen oxide decomposing element 1 (see FIG. 1) of embodiment 1 andthe second electrode 4 and is brought into close contact therewith. Thismixed layer can be formed by dispersing aplatinum group catalyst, acathodic catalyst, a fine-grained electronic conductivity base material,and a fine-grained solid electrolyte film into a solution, heating themto evaporate volatile components, and combining the fine graincomponents with each other.

Since the electrochemical reactions described in embodiment 1 occur atan interface between each electrode and the solid electrolyte film, theamount of reaction and the reaction speed are increased in proportion tothe area of the interface. In embodiment 2, the interface between theelectrode and the solid electrolyte film, that is, the reaction site ofthe electrochemical reaction is increased at the side of the secondelectrode layer 4, so that the efficiency of the electrochemicalreductive reaction of the nitrogen oxide is improved.

EMBODIMENT 3

FIG. 6 is a schematic view showing a nitrogen oxide decomposingapparatus of embodiment 3. In the drawings, same or equivalent parts aredenoted by the same reference numerals and their description will beomitted. The nitrogen oxide decomposing apparatus in this embodimentincludes a nitrogen oxide sensor 14 for detecting the concentration ofnitrogen oxide, and a power source/control device 15 for controlling themagnitude of a current flowing between a first and a second currentlayers 3 and 4 and an energization time in accordance with theconcentration of the nitrogen oxide detected by the nitrogen oxidesensor 14. By this, it is possible to control the magnitude of thecurrent flowing through the nitrogen oxide decomposing element 1 and theenergization time in accordance with the concentration of the nitrogenoxide. Incidentally, it is desirable that the nitrogen oxide sensor 14is disposed in the vicinity of a platinum group catalyst 6 supported bya metal oxide 5 at a cathode side. For example, the nitrogen oxidesensor 14 including a sensitive part having a size of about 1/10 of thenitrogen oxide decomposing element 1 is fixed in the vicinity of themetal oxide 5 and information is extracted to the outside through asignal line.

The operation and effect of the nitrogen oxide decomposing apparatus ofthis embodiment will be described with reference to FIG. 7. In thedrawing, y1 and y2 denote previously set nitrogen oxide concentrationvalues, y1 denotes an energization stop concentration, and y2 denotes anenergization start concentration. That is, at time points t1 and t3 whenthe nitrogen oxide concentration detected by the nitrogen oxide sensor14 exceeds y2, the energization by the power source/control device 15 isstarted, and the decomposition and removal of the nitrogen oxide isperformed. At time points t2 and t4 when the nitrogen oxideconcentration is lowered to y1 or less, the energization is stopped.Thus, only in the case where the nitrogen oxide concentration in theatmosphere becomes high, or the amount of the nitrogen oxide occludedand stored in the platinum group catalyst 6 supported by the metal oxide5 becomes high and the nitrogen oxide concentration of a vapor phase inequilibrium with the amount (occlusion concentration) becomes high, acurrent flows through the nitrogen oxide decomposing element 1, and thenitrogen oxide is decomposed and removed. Further, although not shown,the power source/control device 15 can also operate to increase ordecrease the amount of the flowing current in accordance with theincrease or decrease amount of the nitrogen oxide concentration.

As described above, according to this embodiment, the low-powerconsumption nitrogen oxide decomposing apparatus is obtained which candeal with the concentration change of the nitrogen oxide, and canefficiently use electric energy.

EMBODIMENT 4

FIG. 8 is a schematic view showing a nitrogen oxide decomposingapparatus of embodiment 4 of the invention. This embodiment 4 includes acase 1 shown in FIG. 8(a) and a case 2 shown in FIG. 8(b). Each of thesecases 1 and 2 includes one gas supply port 16 and one gas exhaust port17. Besides, in each of both the cases 1 and 2, a first electrode layer3 and a second electrode layer 4 are disposed to be separate from eachother on one surface 2 a of a solid electrolyte film 2, and a metaloxide 5 and a platinum group catalyst 6 are disposed on the secondelectrode layer 4.

The case 1 of FIG. 8(a) is the case in which similarly to the gases fromthe gas supply ports 9 and 11 of embodiment 1, water vapor (H₂O),nitrogen gas (N₂), nitrogen monoxide gas (NO), oxygen gas (O₂), andhelium gas (He) are supplied from the gas supply port 16, and a reactionwith the water vapor (H₂O) occurs at the side of the first electrode 3as the anode. The case 2 of FIG. 8(b) is the case in which water vapor(H₂O), nitrogen gas (N₂), nitrogen monoxide gas (NO), oxygen gas (O₂),helium gas (He) and hydrocarbon gas (CH₄) are supplied from the gassupply port 16, and a reaction of the hydrocarbon (CH₄) with the watervapor occurs at the side of the first electrode layer 3 as the anode. Inthe drawings, same or equivalent portions are denoted by the samereference numerals and their description will be omitted.

In the nitrogen oxide decomposing apparatus of embodiment 4, the firstelectrode layer 3 and the second electrode layer 4 are respectivelyprovided on the same plane surface of the surface of the solidelectrolyte film 2. The materials constituting the solid electrolytefilm 2 and the first and the second electrode layers 3 and 4 are thesame as those of embodiment 1. As the platinum group catalyst 6supported by hydrogen-terminated zeolite as the porous metal oxide 5,for example, iridium is provided on the second electrode layer 4. Amixed gas of a gas containing water vapor and a gas containing nitrogenoxide is made to come into contact with the first and the secondelectrode layers 3 and 4, so that similarly to the embodiment 1, thenitrogen oxide is decomposed and removed. The reactions at therespective places of the nitrogen oxide decomposing element ofembodiment 4 will be set forth below with respect to the case 1 and thecase 2.

Case 1(anode) 2H₂O→4H⁺+O₂+4e⁻  (chemical formula 12)(cathode) 2NO+4H⁺+4e⁻→N₂+2H₂O  (chemical formula 13)(total) 2NO→N₂+O₂  (chemical formula 14)Case 2(anode) CH₄+2H₂O→8H⁺+CO₂+8e⁻  (chemical formula 15)(cathode) 4NO+8H⁺+8e⁻→2N₂+4H₂O  (chemical formula 16)(total) 4NO+CH₄→2N₂+CO₂+2H₂O  (chemical formula 17)

In the case 1, at the first electrode layer 3 as the anode, as indicatedby the formula (chemical formula 12), the water molecule (H₂O) iselectrolyzed, and hydrogen ions (H⁺) are produced. In the case 2, asindicated by the formula (chemical formula 15), the water molecule andthe hydrocarbon (CH₄) are electrolyzed, and hydrogen ions and carbondioxide (CO₂) are produced. These hydrogen ions pass through the solidelectrolyte film 2 and are moved to the side of the second electrodelayer 4, and as indicated by the formulas (chemical formula 13) and(chemical formula 16), the nitrogen oxide (NO) is reduced and decomposedinto the nitrogen (N₂) and the water molecule (H₂O).

In embodiment 4, in contrast to embodiment 1 (see FIG. 1) in which thespace in the frame 7 is divided into the upper treatment chamber 7 d andthe lower treatment chamber 7 e by the solid electrolyte film 2, thereactions at the anode side and the cathode side occur in the sameprocessing chamber 7 f. Thus, the one gas supply port 16 used both asthe anode gas supply port 9 and the cathode gas supply port 11 can beadopted, and as a result,the anode gas and the cathode gas are mixed.However, since the objective reactions occur by the actions of theanodic catalyst and the cathodic catalyst provided in the respectiveelectrodes, the same effect as the embodiment 1 can be obtained.Further, since the structural parts of the apparatus are decreased, andthe integration of the element on the plane surface becomes possible,the nitrogen oxide decomposing apparatus can be simplified andminiaturized. By this, the installation in the vicinity of the source ofnitrogen oxide becomes possible, and the decomposition and removal canbe efficiently performed in a high concentration area of the nitrogenoxide.

INDUSTRIAL APPLICABILITY

The nitrogen oxide decomposing element of the invention and the nitrogenoxide decomposing apparatus including the same have various uses such asexhaust gas treatment of an automobile, a distributed cogenerationsystem, and air cleaning of an enclosed space such as a long tunnel or afactory.

1-12. (canceled)
 13. A nitrogen oxide decomposing element, comprising: aconductive solid electrolyte film for selectively allowing a hydrogenion to pass through; a first electrode made of an electronicconductivity base material disposed on a part of a surface of theconductive solid electrolyte film and a catalyst for accelerating anodicoxidation; a second electrode made of an electronic conductivity basematerial disposed on the other part of the surface of the conductivesolid electrolyte film and a catalyst for accelerating cathodicreduction; and a platinum group catalyst supported by a porous metaloxide disposed to be adjacent to the second electrode.
 14. The nitrogenoxide decomposing element according to claim 13, wherein the first andthe second electrodes are respectively provided on opposed planesurfaces of the surface of the conductive solid electrolyte film. 15.The nitrogen oxide decomposing element according to claim 13, whereinthe first and the second electrodes are provided on a same plane surfaceof the surface of the conductive solid electrolyte film.
 16. Thenitrogen oxide decomposing element according to claim 13, wherein amixed layer including an electronic conductivity base material, a solidelectrolyte film, a platinum group catalyst and a cathodic catalyst isprovided between the conductive solid electrolyte film and the secondelectrode.
 17. The nitrogen oxide decomposing element according to claim13, wherein the metal oxide is an acidic oxide.
 18. The nitrogen oxidedecomposing element according to claim 17, wherein the metal oxideincludes at least one component of titanium dioxide, zirconium dioxide,aluminum oxide, silicon oxide, magnesium oxide, and tin oxide.
 19. Thenitrogen oxide decomposing element according to claim 13, wherein themetal oxide is an amphoteric oxide.
 20. The nitrogen oxide decomposingelement according to claim 19, wherein the metal oxide includes at leastone component of titanium dioxide, zirconium dioxide, aluminum oxide,silicon oxide, magnesium oxide, and tin oxide.
 21. The nitrogen oxidedecomposing element according to claim 13, wherein the platinum groupcatalyst includes at least one component of platinum, iridium, andpalladium.
 22. A nitrogen oxide decomposing apparatus, comprising: thenitrogen oxide decomposing element according to any one of claims 13 to21 and a frame holding this; a gas supply ports for supplying an anodegas and a cathode gas into the frame; a gas exhaust port for exhaustingthe gases in the frame to outside; and a power source for applying a DCvoltage between the first and the second electrodes.
 23. The nitrogenoxide decomposing apparatus according to claim 22, wherein a gascontaining water vapor is supplied as the anode gas.
 24. The nitrogenoxide decomposing apparatus according to claim 22, wherein a gascontaining nitrogen oxide is supplied as the cathode gas.
 25. Thenitrogen oxide decomposing apparatus according to claim 22, wherein thenitrogen oxide decomposing apparatus further comprises a sensor fordetecting a concentration of nitrogen oxide, and a control device forcontrolling a magnitude of a current flowing between the first and thesecond electrodes and an energization time in accordance with theconcentration of the nitrogen oxide detected by the sensor.
 26. Thenitrogen oxide decomposing apparatus according to claim 25, wherein thesensor is located in a vicinity of the platinum group catalyst supportedby the metal oxide.